1. Field of the Invention
The present invention generally relates to fiber optic interferometric sensors and, more particularly, to extrinsic Fizeau interferometric fiber optic sensors having particular application in hostile environments for dynamic monitoring of strain, temperature or pressure in mechanical structures. The extrinsic fiber optic sensors according to the invention are capable of both relative and absolute measurement of strains and, with suitable modification, can be used for the detection of the relative polarity of dynamically varying strain, temperature or pressure. As used herein, the word "strain" will be understood to mean strain, temperature, pressure, magnetic fields (i.e., magnetostrictive materials), and other like phenomena that can be translated into a displacement depending on the specific application.
2. Description of the Prior Art
Fiber optic Fabry-Perot sensors reported in the literature have been highly sensitive to temperature, mechanical vibration, acoustic waves and magnetic fields. See T. Yoshino, K. Kurosawa, K. Itoh, and T. Ose, IEEE J. Quantum Electron., QE-18, 1624 (1982). Techniques to create the Fabry-Perot cavity have varied from the creation of Bragg gratings in or on the fiber, as described by K. L. Belsley, J. B. Carroll, L. A. Hess, D. R. Huber, And D. Schmadel, in Proc. Soc. Photo-Opt. Instrum., Eng. 566, 257 (1985), to the use of air-glass interfaces at the fiber ends as the reflectors, as described by A. D. Kersey, D. A. Jackson, and M. Corke, in Opt. Comm., 45, 71 (1983). A relatively new technique described by C. E. Lee and H. F. Taylor, in Electron. Lett., 24, 193 (1988), involves fabricating semireflective splices in a continuous length of fiber.
Applied strain at high temperature for intrinsic optical fiber sensors can cause plastic deformation (i.e., creep), making such sensors unsuitable to many hostile environments. The intrinsic Fabry-Perot interferometer described by Lee and Taylor, ibid., exhibits stress-induced birefringence in the air gap which causes polarization changes between the reference and the sensing reflections. This, in turn, causes signal fading observed as a decrease in fringe contrast. Extremely high magnetic fields can also cause polarization changes inside an optical fiber which again would cause a changing state of polarization between the reference and sensing reflections in an intrinsic Fabry-Perot interferometric sensor.
Most Fabry-Perot sensor described in the prior art have been useful in the measurement of quasi-static strain alone. That is, when the induced strain changes its polarity, the Fabry-Perot interferometers would not be able to detect this change if the switch in direction took place at a maximum or minimum of the transfer function curve. Methods for obtaining directional strain information by using thin-film or resistive gauges have been reported by J. Putz, J. Putz, a. Wicks, and T. Diller, in "Thin-film shear stress gauge", presented at the American Society of Mechanical Engineers Winter Annual Meeting, Dallas, Tex., Nov. 26, 1990; however, no corresponding capability has been demonstrated in Fabry-Perot optical fiber sensors in the prior art.